Ductile shear failure damage modelling and predicting built-up edge in steel machining

This paper reports an improved ductile shear failure model for steels and its application, through finite element simulations, to predicting the conditions for built-up edge formation in steel machining. The model has two parts, a standard damage accumulation law and (the improved part) how damage affects the steelʹs flow stress after failure. The accumulation law includes a strain to failure with inverse exponential dependence on hydrostatic pressure and reducing in a blue-brittle temperature range. The flow stress after failure remains finite in compressive hydrostatic conditions, to create a friction resistance to shear across the failure surface. Predictions of built-up edge formation depend strongly on strain hardening behaviour. This affects the hydrostatic stress field in the chip formation region. Simulations show the general features of built-up edge formation (a finite cutting speed range with an upper limit determined by increased ductility with temperature and a lower limit determined, depending on conditions, by insufficient heating for blue-brittleness, lower chip/tool friction or a change to unsteady chip formation). The simulations are tested against previously published observations of built-up edge formation in orthogonal cutting of a Russian steel equivalent to AISI 5130. To extend the work to a wider range of steels requires more data to be gathered on individual steels’ damage accumulation law coefficients. Also, at this stage, the simulations only predict the conditions (cutting speed, uncut chip thickness) in which built-up edge forms. They are not able to follow the growth of the built-up edge to its final shape.